1. Field of the Invention
The invention relates to a method for determining the influencing, caused by an optical system, of the state of polarization of optical radiation to an aberration correction method and to an apparatus, suitable for carrying out such a method, and to a polarization analyser arrangement which can be used in the latter.
2. Description of the Related Art
Various methods and apparatuses are known which can be used to determine how an optical system influences the state of polarization of optical radiation. The term optical system is to be understood in this case as any arrangement of one or more optical components which transmit and/or reflect the incident optical radiation, in particular including lenses and objectives constructed therewith. The term optical radiation is to be understood here as any desired electromagnetic radiation which is applied to the optical system under test, for example visible light or UV radiation. Particularly widely used are ellipsometry methods and ellipsometry apparatuses in diverse forms. In order to describe the state of polarization and how it is influenced or changed by the optical system, use is made of suitable variables such as the Stokes parameters, the Muller matrix, the polarization matrix and the Jones matrix. Reference may be made to the relevant literature for details in this regard.
A method and an apparatus of the type mentioned at the beginning are disclosed in Patent Specification U.S. Pat. No. 5,298,972. In this method and in this apparatus the influencing of the state of polarization caused by an optical system is determined in an integral fashion, specifically by determining a single set of Stokes parameters, assigned to the optical system under test, and determining the Jones matrix resulting therefrom. The radiation is directed via a single-mode fibre in each case onto the optical component and diverted by the latter, the result being to effect spatial beam filtering.
It is also known to utilize polarization effects to produce polarization images of objects. The Patent Specification U.S. Pat. No. 5,396,329 indicates a corresponding image recording system which, in addition to an imaging optics, has an optical retarder, for example in the form of a compensator, and, downstream thereof, a linear polarizer which are both arranged rotatively. Serving as image detection unit is, for example, an imaging camera, a CCD detector or a row of individual detector elements. The computational evaluation is performed via the Stokes parameters and one Muller matrix for each polarization-relevant component.
The Patent Specification U.S. Pat. No. 5,166,752 discloses an ellipsometry system in which a parallel entrance bundle is focused onto an optical system under test such that the individual rays are incident at various angles, and the light cone reflected or transmitted by this optical system under test is refocused into a parallel exit beam. Serving as detector unit is a row of individual detector elements which are struck in each case by light rays which originate from a narrow range of angles of incidence on the optical system under test. The aim of this is to permit simultaneous detection of the state of polarization of light rays incident on the system under test at various angles of incidence without an attendant requirement for a scanning detection operation. This ellipsometry system is used, in particular, to test optical materials for properties which cause a change in state of polarization, specifically the birefringence of an optical volumetric material in the case of transmission measurement.
As is known, it is possible for the purpose of determining the image quality of optics which image with high precision to make use of wavefront sensors with the aid of which deviations of the image-side wavefronts from the ideal imaging behaviour can be determined very accurately. So-called shearing interferometers, for example, are in use for this purpose. A wavefront detection device based thereon is disclosed in Laid-Open Specification DE 101 09 929 A1. This apparatus is also suitable, in particular, for determining the image quality of projection objectives of microlithographic projection exposure machines, and includes means for providing a wavefront source, for example with an optical conductor and a perforated mask arranged at the output thereof, in the object plane of the optical imaging system under test and a diffraction grating in the image plane conjugate to the object plane. Connected downstream of the diffraction grating is a spatially resolving radiation detector, for example in the form of a CCD chip, an interposed optics imaging the interferogram produced by the diffraction grating onto the sensor surface of the detector. This type of wavefront sensor technology can test the imaging system with the aid of the same radiation which is used by the imaging system in its normal operation, and it can be integrated in one component with the imaging system. This type of wavefront sensor is therefore also denoted as an operational interferometer (OI).
In the German Patent Application 102 17 242.0, which is not a prior publication, a measuring apparatus is described which can, in particular, be such an OI apparatus and serves the purpose of interferometric measurement of an optical imaging system which is used for imaging a useful pattern, provided on a mask, into the image plane, the mask being arranged in the object plane for this purpose. It is proposed to implement the wavefront source from the interferometric measurement by means of a measuring pattern formed on the mask in addition to the useful pattern.
A further method, used in practice, of wavefront detection by high-precision imaging systems is represented by point diffraction interferometry, the basic principles of which are described in the relevant specialist literature—see, for example, D. Malacara, “Optical Shop Testing”, Chapter 3.7, John Wiley, New York, 1991. Specific discussions are provided in Patent Specifications U.S. Pat. No. 6,344,898 B1 and U.S. Pat. No. 6,312,373, and in the Laid-Open Specifications JP 11-142291 and WO 02/42728.
In the case of modern high-precision imaging systems of high numerical aperture, used as microlithographic projection objectives, for example, the influence of the imaging system on the state of polarization of the radiation used can scarcely be neglected any longer. Thus, for example, polarization-induced effects on the image quality are produced by birefringence in the case of lenses made from calcium fluoride such as are frequently used for short wavelengths, and by polarization effects at deflecting mirrors. There is therefore a need to be able to determine the influencing of the state of polarization of optical imaging systems of high aperture as well as possible in quantitative terms, in order to draw conclusions on the polarization-dependent image quality.
The invention is based on the technical problem of providing a novel method and a novel apparatus of the type mentioned at the beginning, as well as a polarization analyser arrangement which can be used in this case, with the aid of which the influencing, caused by an optical system under test, of the state of polarization of the radiation used and/or an aberration correction can be determined comparatively accurately such that they are also suitable, in particular, for determining the polarization-induced influence on the imaging quality very precisely in the case of optical imaging systems.